Conventionally, a W-CDMA communications system performs transmission power control for maintaining the communication quality of a dedicated physical channel (DPCH) at a prescribed level.
The transmission power control sometimes performs closed-loop transmission power control in a dual-loop structure of inner-loop control and outer-loop control to maintain stable communication quality under various environments. In this case, the inner-loop control instructs the opposite transmitter to increase or decrease the transmission power to adjust the signal to interference ratio (received SIR) of the radio signal (DPCCH signal) received by the receiver to a target signal to interference ratio (target SIR) as close as possible; and the outer-loop control increases or decreases the target SIR of the inner-loop control to adjust the received block error rate (received BLER: BLock Error Rate) of the received data (user data) decoded from the DPDCH signal in the receiver to the target block error rate (target BLER).
Here, when the transmitter transmits the user data to the receiver intermittently, that is, when the receiver receives the received data (user data or packet data) intermittently via the DPDCH, it cannot measure the received BLER in a section where the user data is not received because of the lack of the data to be measured. In view of this, a method is known of estimating the received BLER required in the outer-loop control in the section where the received BLER cannot be measured.
A proposal is known which uses a fixed value as the target SIR in the received data-less section in the outer-loop control. Besides, another proposal is known which uses the BER of a signal of a dedicated physical control channel (that is, DPCCH signal) for the update control of the target SIR in a transmitted and received data-less section in the outer-loop control. As another calculation method of the BER, a method (re-encoding method) is known which calculates the BER approximately by re-encoding the received data passing through error-correcting decoding and by carrying out bit-by-bit comparison between it and the received data before the error-correcting decoding (see Patent Documents 1 and 2, for example).
FIG. 1 shows a configuration of a conventional mobile station that carries out the outer-loop control using pilot bits of the DPCCH in a section where the user data is not transmitted or received. A radio section 102 performs prescribed receiving processing such as frequency conversion of the signal (received signal) received by an antenna 101. In addition, the radio section 102 performs prescribed transmission processing such as frequency conversion of a DPCH signal from a DPCH spreading section 117 which will be described later, and transmits the DPCH signal passing through the transmission processing via the antenna 101.
A CPICH despreading section 103 despreads the received signal passing through the receiving processing by the radio section 102 by using a spread code assigned to a common pilot channel (CPICH) as shown in FIG. 2A. The CPICH, which is a channel transmitted from the transmitter to the mobile station continuously, includes a known signal. A quality measuring section 105 measures received quality (such as a received bit error rate (BER), a received EC/NO (energy per career to noise power density ratio, or a received RSCP (level)) of the signal (that is, the CPICH signal) passing through the despreading by the CPICH despreading section 103. Typically, the mobile station can carry out communication using the DPDCH by selecting a transmitter to perform communication from among a plurality of transmitters (base stations) based on the received quality of the CPICH signal.
A DPCH modulation section 118 modulates transmission data and a command (that is, TPC bits) for controlling transmission power in the transmitter, which is produced by a TPC bit generating section 114 which will be described later. A DPCH spreading section 117 spreads the TPC bits and transmission data passing through the modulation processing by using a spread code assigned to the DPCH of the mobile station, thereby generating a DPCH signal. In addition, the DPCH spreading section 117 outputs the DPCH signal generated to the radio section 102 described above.
A DPCCH despreading section 106 despreads the received signal passing through the receiving processing by the radio section 102 using the spread code assigned to the mobile station. A DPCCH demodulating section 107 demodulates the signal passing through the despreading by the DPCCH despreading section 106, and generates a demodulation signal including the pilot signal and TFCI signal as shown in FIG. 2B. In addition, the DPCCH demodulating section 107 recognizes from the demodulation signal generated whether the DPDCH signal is transmitted to the mobile station. The DPCCH demodulating section 107 outputs the recognition result to a DPDCH despreading section 109, a DPDCH demodulating section 110, an SIR measuring section 112, and a target SIR calculating section 116.
A pilot error rate measuring section 108 calculates the error rate of the pilot signal included in the received signal demodulated by the DPCCH demodulating section 107, and outputs it to a target SIR calculating section 116.
The DPDCH despreading section 109, when it recognizes from the recognition result from the DPCCH demodulating section 107 that the DPDCH signal is transmitted to the mobile station, despreads the received signal passing through the receiving processing by the radio section 102. The DPDCH demodulating section 110 demodulates the received signal despread by the DPDCH despreading section 109.
A decoding section 111 decodes the received signal demodulated by the DPDCH demodulating section 110, and carries out such processing as error correction, thereby generating received data (user data). In addition, the decoding section 111 outputs the result of the error correction and the like to a error rate measuring section 115.
The error rate measuring section 115 calculates the error rate of the received data after the processing such as the error correction, and outputs it to the target SIR calculating section 116.
The SIR measuring section 112 measures the SIR of the received signal demodulated by the DPCCH demodulating section 107, and outputs it to an SIR comparing section 113.
The SIR comparing section 113 compares the SIR measured by the SIR measuring section 112 with the target SIR calculated by the target SIR calculating section 116, and outputs the compared result to the TPC bit generating section 114.
The TPC bit generating section 114 generates the command for controlling the transmission power of the transmitter in response to the compared result output from the SIR comparing section 113, and outputs it to the DPCH modulation section 118.
The target SIR calculating section 116 calculates the target SIR based on the error rate of the pilot signal calculated by the pilot error rate measuring section 108, the error rate of the received data calculated by the error rate measuring section 115, and the recognition result by the DPCCH demodulating section 107, and updates the target SIR of the SIR comparing section 113.
More specifically, the target SIR calculating section 116 calculates the target SIR by using the error rate of the received data from the error rate measuring section 115 when the DPDCH signal for the mobile station is present, and by using the error rate of the pilot signal from the pilot error rate measuring section 108 when the DPDCH signal for the mobile station is not present, and updates the target SIR of the SIR comparing section 113.
As described above, when the DPDCH signal for the mobile station is not present, updating the target SIR by using the error rate of the pilot signal of the DPCCH enables the estimation of the BLER necessary for the outer-loop control.
However, the outer-loop control using a fixed value as the target SIR in the transmitted and received data-less section presents a problem of causing a failure of the data transmission and reception because of not requiring power in spite of the degradation in quality, or a problem of requiring power even if the quality is good enough.
On the other hand, as for the method that carries out the update control of the target SIR in the transmitted and received data-less section in the outer-loop control using the BER of the DPCCH, when it uses the pilot bits in the DPCCH for calculating the BER, it must take an average over an extended time period to achieve high accuracy because of a small number of pilot bits per radio frame. However, the long time averaging, which has a tendency to deteriorate the trackability to the changes in environments, has a problem of being unable to perform appropriate update of the target SIR when the environments change suddenly. Besides, as for the method (re-encoding method) of calculating the BER approximately by re-encoding the data passing through the error correction decoding, and by carrying out bit-by-bit comparison with the received data before the error correction decoding, it has a problem in that the processing load is heavy in addition to the problem of an error due to uncorrected bits.
Patent Document 1: Japanese patent laid-open No. 2003-318818 (pp. 6-10)
Patent Document 2: Japanese patent laid-open No. 2004-274117 (28th to 35th paragraph, and FIG. 6)